Molecular Formula
A focused revision guide to molecular formulae for Edexcel A Level Chemistry. This page shows how to move from an empirical formula to the actual molecular formula using relative molecular mass, percentage composition and clear worked examples.
What Is a Molecular Formula?
The molecular formula shows the actual number of atoms of each element in one molecule of a compound. It gives more information than an empirical formula because it tells you the real atom count, not just the simplest ratio.
For example, the empirical formula of glucose is CH2O, but the molecular formula is C6H12O6. The molecular formula is six times the empirical formula unit.
Definition: A molecular formula is the actual number of atoms of each element in one molecule of the compound.
Key distinction: an empirical formula gives the simplest ratio, while a molecular formula gives the actual formula of a molecule.
The Core Relationship
To find a molecular formula from an empirical formula, compare the relative molecular mass of the whole compound with the relative formula mass of the empirical formula unit.
n = Mr of compound ÷ Mr of empirical formula unit
Then multiply every subscript in the empirical formula by n. The value of n should be a whole number because one molecule contains a whole-number multiple of the empirical formula unit.
Step 1 — Find the Mr of the empirical formula
Add the relative atomic masses for one empirical formula unit.
Step 2 — Divide the compound Mr by the empirical formula Mr
This gives the multiplier, n.
Step 3 — Multiply the empirical formula by n
Multiply every element subscript by the same multiplier to get the molecular formula.
The molecular formula is found by checking how many times the empirical formula mass fits into the relative molecular mass of the compound.
Worked Example: Empirical Formula Given
Determine the molecular formula of a compound with empirical formula C3H6O and Mr = 116.
Step 1 — Find the Mr of the empirical formula
Mr of C3H6O = 36 + 6 + 16 = 58.
Step 2 — Find the multiplier
n = 116 ÷ 58 = 2.
Molecular formula: C6H12O2. The empirical formula C3H6O has been multiplied by 2.
Shortcut Method from Percentage Composition
If you are given percentage composition and the relative molecular mass of the compound, you can find the molecular formula directly. Instead of first writing the empirical formula, find the mass of each element in one mole of the compound.
For a compound with Mr = 46 containing 52.2% C, 13.0% H and 34.8% O, the percentage tells you what fraction of the 46 g mol-1 belongs to each element.
Step 1 — Find the mass of each element in one mole
Step 2 — Convert each mass into moles of atoms
Molecular formula: C2H6O. This shortcut works because the percentage composition is applied to one mole of compound.
When the Mr Is Not Exact
The relative molecular mass does not always need to match perfectly because experimental data may contain rounding. The important point is that the molecular formula must be a whole-number multiple of the empirical formula.
| Calculation | Likely value of n | Exam decision |
|---|---|---|
| 119.8 ÷ 60 = 1.997 | 2 | Round to 2 because the value is extremely close to a whole number. |
| 178.9 ÷ 30 = 5.963 | 6 | Round to 6 if the data clearly supports a whole-number multiple. |
| 116 ÷ 58 = 2 | 2 | Use 2 directly. |
Exam tip: never multiply only one element. The multiplier applies to the whole empirical formula unit.
Empirical Formula vs Molecular Formula
Different compounds may have the same empirical formula but different molecular formulae. This is why relative molecular mass is needed to identify the actual molecular formula.
| Empirical formula | Empirical formula Mr | Compound Mr | n | Molecular formula |
|---|---|---|---|---|
| CH2O | 30 | 30 | 1 | CH2O |
| CH2O | 30 | 60 | 2 | C2H4O2 |
| CH2O | 30 | 180 | 6 | C6H12O6 |
| HO | 17 | 34 | 2 | H2O2 |
Common Exam Points
- The molecular formula shows the actual number of atoms of each element in one molecule.
- The empirical formula shows the simplest whole-number ratio of atoms.
- To move from empirical formula to molecular formula, you must be given the compound Mr or enough data to work it out.
- Find the Mr of the empirical formula unit before calculating the multiplier.
- The multiplier must be a whole number, allowing for sensible rounding of experimental data.
- Apply the multiplier to every atom in the empirical formula, not just the first element.
- If n = 1, the empirical formula and molecular formula are the same.
- Ionic compounds are normally represented by formula units, so the language of molecular formula is mainly used for covalent molecular substances.
Check Your Understanding
Use these activities to practise converting empirical formulae into molecular formulae and recognising how relative molecular mass controls the multiplier.
QuickSnap
This text summary condenses the page into the essential exam ideas.
- Molecular formula: the actual number of atoms of each element in one molecule.
- Empirical formula: the simplest whole-number ratio of atoms in a compound.
- Multiplier: n = Mr of compound ÷ Mr of empirical formula unit.
- Method: find the empirical formula Mr, calculate n, then multiply every subscript by n.
- Rounding: experimental Mr data may not be exact, but n should be rounded sensibly to a whole number.
- Shortcut method: percentage composition and Mr can be used together to find the mass of each element in one mole of compound.
- Common error: do not multiply only one element. The whole empirical formula unit is multiplied.
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A compound contains carbon, hydrogen and oxygen only. Its percentage composition by mass is shown.
| Element | Percentage by mass |
|---|---|
| C | 40.0% |
| H | 6.7% |
| O | 53.3% |
What is the empirical formula of the compound?
Explain how the volume of hydrogen can be used to calculate the amount of magnesium that reacted.
FAQs
These questions address the most common points of confusion students encounter when converting empirical formulae into molecular formulae.
What is the difference between an empirical formula and a molecular formula?
The empirical formula shows the simplest whole-number ratio of atoms in a compound. The molecular formula shows the actual number of atoms of each element in one molecule. For example, CH2O can be an empirical formula, while C6H12O6 is the molecular formula of glucose.
How do I find the molecular formula from the empirical formula?
Find the Mr of the empirical formula unit, divide the compound Mr by this value, then multiply every subscript in the empirical formula by the answer.
What if the Mr division does not give an exact whole number?
Small differences can occur because of rounding in experimental data. If the value is very close to a whole number, round sensibly. A molecular formula must be a whole-number multiple of the empirical formula.
Can the empirical formula and molecular formula be the same?
Yes. If the multiplier n is 1, the empirical formula and molecular formula are identical. Water is H2O as both its empirical and molecular formula.
Can I find the molecular formula directly from percentage composition?
Yes, if the compound Mr is also given. Find the mass of each element in one mole of compound using the percentage composition, then divide by the relevant Ar values to find the number of atoms of each element.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.